The invention relates primarily to a downhole rotary coring device placeable in a drill string and comprising a head section, a motor, and a core barrel having an outer barrel connected to the motor and an inner barrel placed inside the outer barrel.
Such a rotary coring device is used to obtain a sample of an earth formation.
Known designs use a series of tubes, referred to as drill string, to drill a hole into the formation. The lower end of the drill string is provided with a cutting mechanism, referred to as drill bit, which has a vertical, central hole. When a sample of the formation is required, the drilling is stopped and a coring device incorporating a motor is lowered inside the drill string and secured at the bottom end. By pumping fluid down the drill string, the motor is activated and the coring process is effected.
Rotary coring devices consist of an outer barrel with a coring bit at the lower end, which upon rotation cut an annular hole into the formation. The resulting pillar of rock is entering an inner tube. At the end of the coring process the outer and inner barrel assembly is lifted to break the pillar from the formation and to hoist it to the surface.
In a first aspect of the invention, the motor of the coring device comprises a rotor connected to the outer barrel and a stator connected to the head section, whereby the rotor and the stator are movable with respect to each other in the longitudinal direction of the drill string. In this way, both a rotational movement and a longitudinal movement with respect to the drill string can be performed ensuring an elegant manner for providing the required thrust while saving on space that would otherwise be required for a separate thruster.
A particularly useful manner for implementing such a motor is to select the motor to be of the helical screw type comprising a housing and a helically shaped axis positioned in the housing, whereby said axis is movable longitudinally with respect to the housing.
There are several embodiments feasible for the construction of the rotary coring device that is provided with a motor of the helical screw type each having their own function, advantage and benefit. Said embodiments will be discussed hereinafter with reference to FIG. 6.
Preferably, the downhole rotary coring device according to the invention is characterized in that a rotary bearing connects the inner barrel to the outer barrel, and that the inner barrel is slidably connected to a rod that is fixed to the head section, which rod cooperates with a passage in the inner barrel, whereby the said rod and passage are shaped so as to prevent said inner barrel to rotate. This effectively protects the core that is progressively being cut.
In a further aspect of the invention, on top of the core barrel a chamber is provided for receiving the inner barrel, which chamber is closable with a valve. This offers the advantage that the sample that has been received in the inner barrel, can be secured and safely separated in said chamber from the surroundings.
Again to save on space, it is preferred that in the fully open position the valve is positioned behind a protective sleeve that is in an initial position. The valve need not occupy much place, particularly in the embodiment in which the valve is a curved plate with a circumferential seal. The curvature of the plate then corresponds to the curvature of the barrel and the protective sleeve between which the plate is positioned in the fully open position.
The valve can be reliably operated when the sleeve is provided with lifting balls, and that the inner barrel has an outwardly extending rim suited to cooperate with said balls for lifting the sleeve when the inner barrel is moved into the chamber. When the sleeve is lifted far enough, the valve is no longer prevented from closing, and moves from its open position adjacent to the barrel""s wall, i.e vertically, to a closed horizontal position. This movement from the vertical to the horizontal position can effectively be supported by spring action.
It is further desirable that the chamber is provided with a groove for receiving the lifting balls once the sleeve is placed in a lifted position so as to allow the inner barrel to continue its lifting motion whilst releasing the sleeve to return to its initial position.
In still a further aspect of the invention, the lifting of the inner barrel is supported by providing the rod with a piston that is positioned within the inner barrel, and that adjacent to the piston the rod is having a groove portion for receipt of rotary bearing balls forming part of the rotary bearing connecting the inner barrel to the outer barrel. This construction facilitates that once a complete sample is received in the inner barrel, the piston is located at the uppermost position within the inner barrel, such that the rotary bearing balls of the rotary bearing connecting the inner barrel to the outer barrel are free to leave their connecting position. This allows the inner barrel to move longitudinally with respect to the outer barrel, so that the inner barrel can eventually reach the earlier mentioned chamber in which the sample can be safely secured.
The invention further relates to a rotary coring system comprising a drill string and a downhole rotary coring device as mentioned hereabove, whereby the drill string is suspended from a vessel floating on sea. Such system is intended to take samples from positions below sea level.
The problem in such system is the necessity to make use of vessels floating on the sea, which will consequently move up and down together with the tide and the waves. This may adversely affect the quality of the sample being taken, and in order to prevent these adverse consequences, the rotary coring system according to the invention is characterized in that a frame is positioned and fixed on the sea bed, which is provided with a pipe clamp for the drill string. In this manner, the drill string can be effectively maintained at a steady vertical position without movement up and down due to movements of the vessel from which the drill string is suspended. The drill string can be effectively secured in this manner by having the frame fixed by gravity forces.
It is preferred that the pipe clamp is actuable by hydraulic jacks that are mounted on the frame, and a further preferred embodiment is characterized in that the pipe clamp has rotatable clamping blocks that are movable to and fro the drill string. This allows the drill string to be rotated whilst its vertical position is maintained at the same level.
In some circumstances it is desirable to avail of the possibility to move the drill string intentionally up and down. To that end, it is preferred that the frame has a vertical jacking system for moving the pipe clamp vertically.
The invention and its aspects shall now be further elucidated with reference to the drawing showing non-limiting embodiments of the system and downhole rotary coring device according to the invention.